1. Field of the Invention
The invention relates to a handheld image scanner, more particularly to a handheld image scanner, the scanning movement of which may be manually or automatically controlled.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating an image scanner when in use. Digital scan signals from the image scanner (3) are sent to a computer (2) via an interface device (1). The interface device (1) usually employs a direct memory access (DMA) method to receive the digital scan signals from the image scanner (3). Data from the interface device (1) is then received by the computer (2) for eventual display on a monitor device (not shown) or for eventual storage in a hard disk.
Referring to FIGS. 2 and 3, a conventional handheld image scanner (3) is shown to comprise: a light source (4) (such as a light emitting diode (LED) array or a cold cathode fluorescent tube); a horizontal line scanning means including a reflex (5) which is inclined at a 45.degree. angle and a lens and image sensor (6); a transverse distance detecting means including a roller (7), a transmission means (8), and a signal generating means including a perforated rotary plate (9) and a photoemitter and sensing means (10); a brightness control knob (11); and the associated variable resistors, printed circuit board and housing.
When the conventional handheld image scanner (3) is in use, the light source (4) emits light so as to illuminate a portion of an image being scanned. Various light signals are generated according to the different tones on the image. The light signals are transmitted to the reflex mirror (5) inside the housing and are deflected towards the lens and image sensor (6) to thereby produce an electric signal which varies according to the intensity of light signal received. The image sensor has a plurality of light sensitive elements and is capable of scanning at resolutions greater than 200 dots per inch (dpi).
The image sensor can scan only one horizontal line of predetermined dimensions at one time. Thus, the handheld image scanner (3) must be moved in a transverse direction so as to scan the succeeding lines.
Referring to FIG. 4, the roller (7) rolls along the surface of the image being scanned when moving the image scanner (3). Rotation of the roller (7) drives the transmission means (8) to correspondingly rotate the rotary plate (9). The rotary plate (9) is formed with a series of radially extending notches. The photoemitter and sensing means (10) includes a C-shaped seat, a light emitting means (not shown) mounted on an inner side of the C-shaped seat and a phototransistor (not shown) mounted opposite to the light emitting means. The rotary plate (9) extends into an opening in the C-shaped seat between the light emitting means and the phototransistor. The photoemitter and sensing means (10) can thus generate a series of ON/OFF signals whenever the rotary plate (9) is in motion. The ON/OFF signals are received by the computer (2) and are indicative of the distance traversed by the image scanner (3).
Manual movement of the conventional image scanner (3) in a transverse direction relative to the image has the following drawbacks:
1. Unsteady hand movement and slight inclined movement of the image scanner (3) can cause distortions in the resulting image transduced by the lens and image sensor (6).
2. Different degrees of resolution require different scanning speeds. A table of resolution (in dpi) versus the preferred scanning speed (in inch/sec) is shown below (Data is obtained under the condition of each line being exposed to light for a period of 3.57 milliseconds):
______________________________________ Resolution 100 200 300 400 600 800 Scanning Speed 2.8 1.4 0.93 0.7 0.46 0.35 ______________________________________
It is difficult to obtain the preferred scanning speed for a desired degree of resolution when movement of the image scanner (3) is manually controlled. Note that high degrees of resolution require low scanning speeds. However, steady hand movement is difficult to attain at such low speeds. Furthermore, no useful information can be obtained when the hand moves at a speed much faster than the preferred scanning speed. 3. When a large volume of scan data is available, the memory capacity of the computer (2) is immediately filled and further scan data cannot be accommodated in the memory unit. Data in the memory unit should be first transferred to a hard disk so as to permit the computer (2) to receive additional scan data. However, since the manual scanning action is continuously performed, it is possible that some scan data will be lost while the computer (2) transfers data to the hard disk.